Hudson appears to use an included library of log4j instead of the log4j library included in the project to build. The version of this library is 1.2.9. The current stable release is 1.2.15.
This can raise errors when using methods added after 1.2.9. An example:

Caused by: org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [org.jbpm.pvm.internal.cfg.SpringConfiguration]: Constructor threw exception; nested exception is java.lang.NoSuchMethodError: org.apache.log4j.Logger.trace(Ljava/lang/Object;)V
Caused by: java.lang.NoSuchMethodError: org.apache.log4j.Logger.trace(Ljava/lang/Object;)V
at org.jbpm.internal.log.Log4jLog.trace(Log4jLog.java:72)

I have done a quick test by replacing the log4j library in the hudson.war file and at first glance it seems to be working (no thorough test). The origin of this problem is cobertura. When starting junit with cobertura for code coverage, hudson is using the lib from cobertura instead of the lib included in the build project.